TCP/IP Tutorial and Technical Overview - IBM Redbooks

3.1.3 IP routing
► Subnet 4: 30 hosts
► Subnet 5: 30 hosts
This cannot be achieved with static subnetting. For this example, static
subnetting divides the network into four subnets each with 64 hosts or eight
subnets each with 32 hosts. This subnet allocation does not meet the stated
requirements.
To divide the network into five subnets, multiple masks need to be defined. Using
a mask of 255.255.255.192, the network can be divided into four subnets each
with 64 hosts. The fourth subnet can be further divided into two subnets each
with 32 hosts by using a mask of 255.255.255.224. There will be three subnets
each with 64 hosts and two subnets each with 32 hosts. This satisfies the stated
requirements and eliminates the possibility of a high number of wasted host
addresses.
Determining the subnet mask
Usually, hosts will store the subnet mask in a configuration file. However,
sometimes this cannot be done, for example, as in the case of a diskless
workstation. The ICMP protocol includes two messages: address mask request
and address mask reply. These allow hosts to obtain the correct subnet mask
from a server (refer to “Address Mask Request (17) and Address Mask Reply
(18)” on page 116).
Addressing routers and multihomed hosts
Whenever a host has a physical connection to multiple networks or subnets, it is
described as being multihomed. By default, all routers are multihomed because
their purpose is to join networks or subnets. A multihomed host has different IP
addresses associated with each network adapter. Each adapter connects to a
different subnet or network.
An important function of the IP layer is IP routing. This provides the basic
mechanism for routers to interconnect different physical networks. A device can
simultaneously function as both a normal host and a router.
A router of this type is referred to as a router with partial routing information. The
router only has information about four kinds of destinations:
► Hosts that are directly attached to one of the physical networks to which the
router is attached.
► Hosts or networks for which the router has been given explicit definitions.
Chapter 3. Internetworking protocols 77